As for such a compact valve, it is conventionally known that an actuator for a valve body that opens and closes an orifice is formed of a shape memory alloy, and the valve body is displaced by applying electric current to the actuator, as shown in Japanese Laid-Open Patent Publication No. 05-99369, for example. In such a control valve, in order to increase sealing property of the orifice, it is necessary to increase a pressure applied to the orifice, so that the shape memory alloy serving as the actuator contracts after contacting the orifice and further pressurizes the orifice in closing operation of the valve. In addition, the orifice is additionally pressurized due to variation of the shape memory alloy in manufacturing process thereof. The shape memory alloy receives a stress (a load) from the orifice that is fixed on the valve body. The stronger the pressure to the orifice from the shape memory alloy becomes, the larger the stress becomes.
Generally, a shape memory alloy which fevers by energization has repeatability that it takes an extended shape at normal temperature, fevers by energization, takes a previously memorized contracted shape when a temperature thereof becomes higher than a certain temperature value, and returns to the original shape when the temperature thereof returns to the normal temperature. However, when an overload is applied to the shape memory alloy in a state of memorized shape, the repeatability of recovery may be damaged, and thus, it cannot return to the shape originally memorized due to deterioration with age by repetition of the overloads.
On this account, in a valve using a shape memory alloy as an actuator, when repeatability of recovery of the shape memory alloy is deteriorated, control performance of the valve falls off. In addition, the valve shown in above mentioned Patent Publication needs two shape memory alloys as the actuator, so that configuration thereof becomes complex and it causes high cost of production.
In addition, as shown in Japanese Laid-Open Patent Publication No. 09-313363, for example, there is a valve in which a coil spring formed of a shape memory alloy extends when a temperature thereof becomes above a predetermined value, and overcomes a biasing force of a biasing spring, so that a valve body in open state is pressurized to be closed. Since the shape memory alloy is formed in the coil spring shape, so that a profile of the valve becomes larger, and thus, downsizing of the valve is difficult. In addition, when thickening a diameter of a wire of the coil spring to obtain a predetermined force, responsiveness in heat radiation becomes worse due to increase of heat capacity of the coil spring. In addition, as shown in Japanese Laid-Open Patent Publication No. 11-153234, there is a valve that uses a wire formed of a shape memory alloy to perform closing operation. Since temperature distribution occurs in a folded portion of the wire, durability of the valve is easily deteriorated by thermal stress. When the wire works to operating limit, the shape memory alloy suffers from overloads, and thus, deterioration with age occurs.